The study underlines the importance of early antiviral treatment to stop the viral population explosion. The authors, led by Menno de Jong of the Oxford University Clinical Research Unit in Ho Chi Minh City, Vietnam, suggest that treatment to blunt the late-stage immune response should also be explored.

"Our observations point to a central role for high viral burden in the pathogenesis of human H5N1 disease and suggest that timely suppression of viral replication should remain the mainstay of treatment of influenza H5N1," states the report, published online yesterday by Nature Medicine.

The study has also prompted experts to mention the possibility of using intravenous antiviral treatment in an effort to move drugs to the sites of viral activity faster than is possible with oral drugs like oseltamivir (Tamiflu).

De Jong's team conducted virologic and immunologic studies on 18 H5N1 patients and eight patients with ordinary seasonal flu (H3N2 and H1N1 viruses) in 2004 and 2005. Thirteen of the 18 H5N1 patients died of the illness. The H5N1 patients presented for treatment an average of 6 days after the onset of illness—well beyond the recommended 2-day window for starting antiviral treatment. The researchers looked for the virus in samples from the nose, throat, blood, and rectum.

Among the virologic findings:

H5N1 patients had more viral material (viral RNA) in the throat than in the nose, and they had more viral RNA in the throat than patients with ordinary flu had.

H5N1 patients who died had the highest levels of viral RNA.

Viral traces were found in blood samples from 9 of the 16 H5N1 patients whose blood was tested; viral RNA in blood was associated with high viral loads in throat specimens.

Viral RNA was found in rectal samples from 5 of 7 H5N1 patients, and three of these had diarrhea.

The researchers also examined the blood levels of seven cytokines and chemokines—molecular messengers that call various kinds of immune cells into action, triggering inflammation. They found that H5N1 patients had significantly higher levels of 6 out of 7 of these substances than seasonal flu patients had. In addition, levels of four chemokines were particularly high in H5N1 patients who died.

"Our observations indicate that high viral load, and the resulting intense inflammatory responses, are central to influenza H5N1 pathogenesis," the authors write. "The focus of clinical management should be on preventing this intense cytokine response, by early diagnosis and effective antiviral treatment."

They add that the limited effectiveness of antiviral treatment in H5N1 patients when started late may reflect the inability of the drugs to stop the cytokine storm at that point. At that stage, they suggest, treatment to limit or change the immune response "has potential benefits." But they emphasize that the main focus should be on early diagnosis and antiviral treatment.

The central question raised by the study, according to infectious disease expert Michael T. Osterholm, PhD, MPH, is, "At what level of viremia does the cytokine storm get triggered? Once that happens, does it matter what the ongoing level of viral activity happens to be? Once the cytokine storm begins, it's already too late to have any impact with antivirals."

Osterholm, who is director of the University of Minnesota Center for Infectious Disease Research and Policy, publisher of the CIDRAP Web site, said he doesn't know of anyone who has tried immunosuppressive or anti-inflammatory drugs to stop the cytokine storm in H5N1 patients.

"No one knows what the consequences of that might be," he said. Reducing the immune response could backfire by allowing the virus to run wild, he suggested.

Virologist Frederick G. Hayden, MD, said intravenous administration might improve the effectiveness of antiviral treatment in H5N1 disease, according to a Canadian Press report published yesterday.

"A potent parenteral agent . . . is really needed and will give us the ability, I hope, to more rapidly control replication in patients with these kinds of severe infections," Hayden was quoted as saying.

Hayden, of the University of Virginia in Charlottesville, heads a World Health Organization research network that plans to assess various treatment regimens for H5N1 patients, the story said. The report said injectable forms of two antivirals—peramivir and zanamivir (Relenza)—are in development.

Osterholm said intravenous drugs move into the system faster, but giving drugs intravenously is harder than giving them orally. In a flu pandemic, he said, "Will there be IV bags available? If you give a drug that's only IV, and then you run out of IV sets, you've got a problem."